Breast cancer is the leading site of new cancer cases in women, and the second leading cause of death. Considerable progress has been made in determining the events in the initiation and early development of breast cancer. However, relatively little is known about the molecular basis of metastasis, the primary cause of death from this cancer. Tumor progression to the metastatic state involves important interactions between the tumor cell and its environment. To understand these interactions it has become clear that a more complete knowledge of normal mammary gland development is also required. Recently, we have shown that macrophages are recruited to the stroma surrounding the developing mammary epithelium and infiltrate into mammary tumors. Using mouse mutants that deplete the number of macrophages owing to a null mutation in the gene for major growth factor for macrophages, Colony Stimulating Factor-1 (CSF-1), we found that these cells are required both for mammary development and for tumor progression to the metastatic state. These data suggest that many of the processes, modulated by macrophages that are required for the outgrowth of normal epithelium through the fat pad, are recapitulated during tumor invasion. The current application aims to understand the mechanistic basis of macrophage function in tumor progression and normal mammary development. It also aims to identify the means whereby alterations in the stromal compartment potentiates tumor progression to the metastatic state. The specific aims are: 1) Role of CSF-1 regulated macrophages in mammary gland development and tumor progression. In this aim, the generality of the findings will be extended to other mouse models of breast cancer and the requirement for CSF-1 signaling in mammary development and tumor progression determined. 2) Isolation of novel genes associated with macrophage action in mammary development and tumor progression. This aim will use state-of-the-art methodologies in proteomics and genomics to identify novel genes that may have a causal role in the macrophage enhancement of mammary gland development and tumor genesis. 3) The mechanistic basis of the role of macrophages in mammary gland development and tumor genesis. This will explore the function of genes isolated in aim 2 as well as determine the involvement in tumor progression and mammary development of candidate genes, that affect angiogenesis and which are known to be synthesized by macrophages. In humans, over-expression of CSF-1 and CSF-1R in tumors of the breast is associated with poor prognosis. Our studies suggest that CSF-1 regulates a population of macrophages that play an important role in tumor progression to the metastatic state. Understanding the mechanistic basis of the macrophage enhancement of metastasis could result in the development of novel therapeutics aimed at to prevent this step.